Mechanical contraction of the heart occurs during which of the following phases of the cardiac cycle?
a. Phase 0 c. Phase 3
b. Phase 2 d. Phase 4
B
During phases 1 and 2, an electrical plateau is created, and during this plateau, mechanical contraction occurs. Because there is no significant electrical change, no waveform appears on the electrocardiogram (ECG). During phase 0 (depolarization), the electrical potential changes rapidly from a baseline of ?2-90 mV to +20 mV and stabilizes at about 0 mV. Because this is a significant electrical change, it appears as a wave on the ECG as the QRS. During phase 3 (repolarization), the electrical potential again changes, this time a little more slowly, from 0 mV back to ?2-90 mV. This is another major electrical event and is reflected on the ECG as a T wave. During phase 4 (resting period), the chemical balance is restored by the sodium pump, but because positively charged ions are exchanged on a one-for-one basis, no electrical activity is generated, and no visible change occurs on the ECG tracing.
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